This invention relates to the ultrasonic detection of hydrogen damage in boiler tubes and, more particularly, to a pitch-catch method of transmitting angle-beam ultrasonic shear-waves axially and circumferentially of the axis of a boiler tube to detect hydrogen damage.
In recent years, a number of boiler tube failures have been attributed to hydrogen damage. Hydrogen damage is caused by the diffusion of hydrogen through steel reacting with carbon to form methane, which builds up local stresses at the grain boundaries, forming microfissures that propagate radially from the inner waterside surface of the tube toward the fireside surface.
Hydrogen damage is usually associated with a corrosive process and some type of triggering mechanism such as overheating. The hydrogen typically forms under a corrosion deposit. Hydrogen is formed by a corrosive reaction, between the tube metal and the corrosion deposit, that acts as a triggering mechanism which causes water to dissociate into hydrogen. The hydrogen more readily diffuses into the metal through the corrosion deposit. Once in the metal, the hydrogen interacts with carbon in the steel to form methane. The larger methane molecules cannot diffuse out of the metal. Consequently, high pressure areas are formed which create microfissuring at the grain boundaries and, ultimately, a form of tube failure, known as blowout, that is characterized by a thick-lipped fracture.
Some ultrasonic techniques have been used to detect hydrogen damage or corrosion in fossil boiler tubes with only limited success. Known ultrasonic techniques, however, have been unable to detect hydrogen damage underneath welds and field tests indicate that damage underneath the weld does not always extend beyond the weld itself. The inability to detect localized areas of hydrogen damage underneath welds has been a major concern. Visual inspection of welds or total replacement of tubes with a failure history, for this reason, have been undertaken at great inconvenience and expense to avoid tube failures due to hydrogen damage at weld areas.